1. Field
The present invention generally relates to the field of wireless communication systems. More specifically, the invention relates to adaptive antenna array techniques for code division multiple access communication systems.
2. Related Art
In wireless communication systems several users share a common communication channel. To avoid conflicts arising from several users transmitting information over the communication channel at the same time requires the use of some form of multiple access protocol, such as Code Division Multiple Access (CDMA). In addition to providing multiple access allocation to a channel of limited capacity, a protocol can serve other functions, for example, providing isolation of users from each other, i.e. limiting interference between users, and providing security by making interception and decoding difficult for a non-intended receiver, also referred to as low probability of intercept.
In CDMA systems each signal is separated from those of other users by coding the signal. Each user uniquely encodes its information signal into a transmission signal. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the information. The encoding of the information signal spreads its spectrum so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the information signal. For this reason CDMA is also referred to as “spread spectrum” modulation or coding.
The energy of each user's signal is spread across the channel bandwidth so that each user's signal appears as noise to the other users. So long as the decoding process can achieve an adequate signal to noise ratio, i.e. separation of the desired user's signal from the “noise” interference of the other users' signals, the information in the signal can be recovered. Other factors which affect information recovery of the user's signal are different conditions in the environment for each subscriber, such as fading due to shadowing and multipath. Briefly, shadowing is interference caused by a physical object interrupting the signal transmission path between the transmitter and receiver, for example, a large building. Multipath is a signal distortion which occurs as a result of the signal traversing multiple paths of different lengths and arriving at the receiver at different times. Multipath is also referred to as “time dispersion” of the communication channel. Multipath fading may also vary with time. For example, the amount of multipath fading can vary rapidly as a result of the signal reflecting off of moving objects such as cars or trees blowing in the wind.
In wireless communications, especially in voice communications, it is desirable to provide communication between two users in both directions simultaneously, referred to as duplexing or full-duplexing. One method used to provide duplexing in CDMA systems is frequency division duplexing. In frequency division duplexing, one frequency band is used for communication from a base station to a user or “subscriber unit”, called the “forward” channel, and another frequency band is used for communication from the user or subscriber unit to the base station, called the “reverse” channel. A forward channel may also be referred to as a “downlink” channel, and a reverse channel may also be referred to as an “uplink” channel or a “return link”. Specific implementation of coding and modulation may differ between forward and reverse channels.
One collection of techniques which can be used to increase signal reliability is referred to as “power control”. Simply stated, power control adjusts the power of the signal at the transmitter while the signal is being transmitted in order to compensate for varying conditions in the communication channel, such as relative movement of different users and multipath fading. One object of power control is to keep the power transmitted to each user at a minimum level needed to maintain a reliable communication link, thereby minimizing the interference between different users' signals discussed above. For example, power control can be implemented on a forward link by having each user receiving unit periodically send power control information to the base station transmitter. In one typical CDMA system, the power control information is in the form of a bit which is sent every 1.25 milliseconds. For example, if the bit has values of +1 and −1, the +1 can be interpreted as information that the receiver has a weak signal, i.e. the receiver needs more power transmitted to it, and the −1 can be interpreted as information that the receiver has a strong signal, i.e. the receiver can get by on less power transmitted to it. Thus, the base station can adjust the signal power transmitted to each user separately in order to minimize the interference between users by optimizing the signal power for each user.
Adaptive antenna array technology can also be used to minimize the interference between users. Adaptive antenna array technology can be used to focus and direct the energy in a transmitted signal into a “beam” which can be used to dramatically increase the efficiency of signal transmission. The shape of the beam can be altered. For example, the beam can be made narrow or wide. The beam can also be “pointed” or “steered” in different directions. The particular shape and direction of a beam transmitted from an antenna array is referred to as the “antenna beam pattern”. By narrowing the antenna beam pattern directed to a specific user, interference is reduced or eliminated for other users which are outside the specific user's antenna beam pattern, thereby helping to minimize the interference between users. As interference between users is reduced, system capacity is increased, that is, a greater number of users are able to simultaneously use the system.
There are a number of approaches for using adaptive antenna array technology in wireless communication systems. Some of the approaches are described in U.S. Pat. No. 6,006,110, entitled “Wireless Communication Network Using Time-Varying Vector Channel Equalization For Adaptive Spatial Equalization” and assigned to Cisco Technology, Inc., San Jose, Calif.
In one approach for using adaptive antenna array technology at the base station, the base station estimates the angle of arrival of the signal received from the user. The base station then uses the angle of arrival estimate to form an estimate for an angle of transmission. With frequency duplexing, as discussed above, the received signal and the transmitted signal are on different frequencies, so that the radio frequency channel differs for the two signals. Depending on the degree of correlation in direction between the two channels at different frequencies, the estimate for the angle of transmission is a better or worse approximation. To the extent that the approximation is inaccurate, a wider antenna beam pattern must be used, limiting the reduction in interference between users with this approach.
In a second approach, the base station transmits an orthogonal pilot signal on each antenna element of the adaptive antenna array. The user's subscriber unit determines the amplitude and phase of each pilot signal, and the subscriber unit reports that information back to the base station on the return link. The base station uses the information to form a narrow beam directed toward the user. This approach requires extra bandwidth on the return link to transmit the pilot signal amplitude and phase information, i.e. non-user information, on the return link, i.e. the reverse channel. To the extent that channel capacity is used to transmit non-user information, less channel capacity is available for transmitting user information. In other words, an “overhead” is incurred for signal transmission on the return link, and the effective bandwidth efficiency of transmission on the return link is reduced. Moreover, this approach entails added complexity at the subscriber unit, which can involve added cost for the subscriber unit. Thus, this second approach is not “backward compatible” in the sense that introducing it into an existing CDMA communication system would require modification or replacement of a large number of subscriber units for the improvement to be effective.
Thus, there is a need in the art for reducing interference between users and for increasing system capacity by providing signal transmission using accurate antenna beam patterns. There is also a need in the art to form accurate antenna beam patterns without incurring signal transmission overhead on the return link. Further, there is a need in the art to provide signal transmission systems using accurate antenna beam patterns which are backward compatible with existing systems and which do not increase the complexity and cost of subscriber units.